Add support for handwritten custom_code folder.

[rileykarson]

This includes support for copied + compiled code, as well as expanding subdirectories into the provider directory. This means the MM source can be split by service, while the generated code will all be in one folder, saving us some time if (likely when) we split the provider into service packages.

If this PR is for Terraform, I acknowledge that I have:

• [ ] Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• [ ] Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• [ ] Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• [ ] Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• [ ] Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 31 insertions(+), 23 deletions(-)) Terraform Beta: Diff ( 3 files changed, 31 insertions(+), 23 deletions(-)) TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2022 Passed tests 1793 Skipped tests: 226 Failed tests: 3

Action taken

Triggering VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccServiceNetworkingPeeredDNSDomain_basic|TestAccFirebaserulesRelease_BasicRelease|TestAccCloudAssetFolderFeed_cloudAssetFolderFeedExample

[modular-magician]

Tests passed during RECORDING mode: TestAccCloudAssetFolderFeed_cloudAssetFolderFeedExample[view] TestAccServiceNetworkingPeeredDNSDomain_basic[view] TestAccFirebaserulesRelease_BasicRelease[view]

All tests passed View the build log or the debug log for each test